In general, a coated copper wire for forming a coil by being wound on a bobbin, etc., often has a circular cross section in a direction orthogonal to the longitudinal direction. In contrast, in recent years, a wire member called a regular-square wire has been developed. The regular-square wire refers to a wire member having a cross section with substantially a horizontal-to-vertical ratio of 1:1, that is to say, having a cross section almost regular square in shape. It is known that a coil having an extremely excellent characteristic can be obtained by winding such a regular-square wire to form a coil.
That is to say, when a conventionally-used wire member having a circular cross section is wound on an outer periphery of a bobbin, a gap inevitably occurs no matter how closely the wire member is wound because of its geometrical shape. In contrast, a regular-square wire can be wound in a plurality of tiers on a bobbin virtually with no gap. Accordingly, the coil space factor of a wire having a circular cross section is about 90.6% at the maximum (that is to say, gaps become 9.4%). In contrast, for a regular-square wire member, a coil space factor of nearly 100% can be obtained. Thus, in order to obtain the same magnetic field by turning on electricity of the same current, it is possible to make the volume of a coil smaller.
Furthermore, in the case of a coil formed by winding a wire having a circular cross section, as is apparent from the above-described space factor, since a heat-insulating air layer occupies about 10% in cross-sectional area ratio of the tiered wire, the thermal conductivity from the inside of the coil is low. Thus, there is a problem in that the maximum current that can be supplied is limited by the heat-generation value of the coil. In contrast, in the case of a regular-square wire, the side surfaces of the adjacent wire member are closely adhered with each other at winding time, and thus an air layer in the winding wire can be substantially zero. It is therefore possible to maintain high thermal conductivity, and thus there is an advantage in that the electric current supplied can be increased. Accordingly, it becomes possible to reduce the volume of a coil to obtain the same magnetic field.
Incidentally, in order to make the most of the characteristic of a wire member having a rectangular cross section such as the regular-square wire described above, the wire member needs to be wound on the outer periphery surface of the bobbin with no gap therebetween. In a conventional wire-winding apparatus, the winding of a wire has been possible without requiring an accurate guide in particular, because the apparatus winds the wire member having a circular cross section. However, in the case of a wire member such as a regular-square wire, when winding is performed by the conventional wire-winding apparatus at a high speed, a side surface of the wire member wound on the outer periphery of the bobbin and the opposed side surface of the wire member adjacently wound separate in the axial direction of the bobbin, or twist with each other. Thus, even if the entire length of the cylindrical part of the bobbin is fabricated to meet the width of the wire member and a predetermined turns of winding with high precision, the winding for one tier might be completed without reaching a predetermined number of turns. In such a case, an air layer is formed inside the wound wire, and thus the advantage of the coil using a regular-square wire is lost.
Furthermore, in the case of forming a coil using a regular-square wire, it is a very important problem how to correctly fold back the wire at both ends of the coil. That is to say, when a wire is continuously wound in a plurality of tiers, in order to form a stable coil, it becomes very important whether or not the winding is reliably performed with high precision while suppressing gaps as much as possible when switching from a lower tier to an upper tier. In order to achieve this, it becomes necessary to increase wire-winding precision in each tier and to make the condition at each folding back timing the same as much as possible.
In order to cope with such a problem, a patent document (Japanese Unexamined Patent Application Publication No. 2000-114084) has disclosed a technique in which a wire member is wound on a bobbin while being moved to one side of the flange of the bobbin using an inclined thin film by winding the wire member on a thin film disposed on an outer periphery of the bobbin.
However, according to such a conventional technique, in the case of disposing a thin film on the outer periphery of the bobbin, it is necessary to provide equipment such as a cut-and-hold mechanism dedicated for the tin film. This causes an increase in cost and results in a complicated wire-winding apparatus. Also, there is a problem in that the cost of the thin film is added and the unnecessary thin film remains in the coil as a product to deteriorate the appearance of the product. Furthermore, there is also a problem in that it is difficult to wind the wire in alignment if the wire is not properly disposed at the beginning time of winding a regular-square wire.